The invention relates generally to the measurement of film thicknesses. The invention relates particularly to such measurement using an electron analyzer.
Many technologically advanced devices rely upon composite structures having a very thin, substantially planar film covering a substrate of another material. An example of such a device is a magnetic recording or read head which has an active surface layer of a ferromagnetic material. High-performance ferromagnetic materials based on, for example, heavier elements such as cobalt, are often brittle and subject to oxidation so that it is common practice to cover the ferromagnetic layer with a very thin protective layer, often of a carbon-based material such as diamond. However, the performance and durability of such devices depend on the manufacturing process to produce a thickness of the covering layer within a relatively narrow range. If the structure is an electromagnetic sensor, the coating thickness must be closely controlled so as to not degrade the sensor performance.
Many thickness measurement techniques are available to measure such film thicknesses, which often are in the range of 1 to 150 nm. Optical techniques are simple, but their dependence on optical properties of both the film and substrate preclude their use for some combination. Auger electron spectroscopy, to be described below, is commonly used for compositional control and in principle can be used for measuring film thickness. However, it is entirely too slow for use in a production environment if the film thickness exceeds 3 nm and is practically useless at thicknesses above 5 nm. Scanning electron microscopy is straightforward, but it is a very slow process and requires the sectioning of the sample being tested. Frequent sampling in a production environment requires a fast, non-destructive technique.
Electron spectroscopy is a well known technique for characterizing the atomic constituents in a solid. Briggs et al. have edited a complete reference in Practical Surface Analysis, vol. 1, Auger and X-ray Photoelectron Spectroscopy, 2nd ed., (Wiley, 1990). In the typical practice of Auger spectroscopy, the solid is probed with an electron beam in the low keV range of energies and produces a secondary electron through an Auger transition process having a well defined Auger energy EAUGER. In Auger spectroscopy, the probing radiation ejects an inner-shell electron from an atom. Then in the Auger transition, a first outer-shell electron falls into the inner-shell vacancy and a second outer-shell electron is ejected carrying the difference in energy. The spectrometer analyzes the energy of the ejected electron as the Auger energy EAUGER. The Auger energy EAUGER is for the most part unique for each atom, primarily dependent upon the atomic number Z. Thus, the measured electron energy can be used to determine the composition of the material, at least near its surface. These energies are generally in the range of a few hundred eV to a few keV for the typical practice of Auger electron spectroscopy. Usually to enhance the Auger signal, the primary energy Ep is made twice or more the Auger energy EAUGER. Auger electron spectroscopy allows the very quick and highly accurate measurement of film thicknesses up to about 30 nm. Other types of electron spectroscopy are possible with similar equipment, and the technology is close to electron microscopy.
A generic electron spectrometer is schematically illustrated in FIG. 1. Other geometrical relationships may be used. An electron gun 10 emits a primary radiation beam 12 of energy Ep towards a sample 14 under test, which is supported on a holder 15. An electron energy analyzer 16 receives a beam 18 of secondary electrons emitted from the sample 14 and characterized by energy Es. The low electron energies require that the entire analyzer be operated at very high vacuum levels. The secondary beam 18 tends to be spatially very broad. The electron energy analyzer 16 typically has a spatially fixed entrance slit 20 to fix the angle between the analyzer 16 and the sample 14, and it internally analyzes the secondary energy Es by means of a electrostatic retarder or a magnetic analyzer or other means. Although in some automated applications, the electron analyzer 16 outputs a small number of experimentally determined parameters, the typical analyzer at some level outputs an energy spectrum from which the energy location of one or more peaks is extracted. Such electron spectrometers are well known, very often as Auger or ESCA spectrometers, and are commercially available from several sources, including Physical Electronics (PHI), a division of Perkin-Elmer of Eden Prairie, Minn., Vacuum Generators of the United Kingdom, and Omicron of Delaware.
Although electrons are often used as the probing radiation, other types of radiation can By produce similar effects, for example, X-ray or positrons.
One of the major experimental effects in electron spectroscopy is background noise introduced by inelastic scattering of the primary electrons (if an electron source is used) and of secondary electrons as they pass through the material between its surface and their points of interaction with the constituent atoms of the material. All electrons experience both elastic and inelastic collisions. Inelastically scattered electrons have a wide distribution of energies beginning at the energy Ep of the probing beam and extending downwardly.
Primary electrons used for Auger spectroscopy typically have energies of a few keV while the Auger transitions are typically below 1 keV. A 1 keV electron has a mean free path in a solid of about 3 nm; a 3 keV electron, 15 nm. X-rays exhibit significantly deeper penetration. Furthermore, secondary Auger electrons are subject to the same type of inelastic scattering. Many technical articles have attempted to explain and quantify the effects of inelastic scattering in order to extract the Auger spectrum. Other technical articles have used the inelastic spreading as a way of measuring the scattering cross-sections between electrons. Elastic scattering depends upon the average atomic number Z of the material and is stronger in materials with higher Z.
A method and apparatus for quickly and non-destructively determining the thickness of an overlayer of one material formed over an underlayer of another material having a different effective atomic number. A source of primary radiation, for example, keV electrons or X-rays, creates a wide spectrum of inelastically scattered secondary electrons. The spectrum of secondary electrons emitted through the overlayer of a test sample is measured and compared to similar spectra for reference samples having known thicknesses of the overlayer to thereby determine the overlayer thickness in the test sample. The apparatus may be derived, in the case of electrons being the primary radiation, from conventional electron spectrometers.
In one embodiment of the invention, ratios of the intensity of a portion of the spectrum of inelastically scattered electrons to that of elastically scattered electrons are measured both for the reference and test samples.